"Join as Shape" error when trying to import a file for shape keys from ZBrush
Okay, so I found an answer. To reiterate, the problem was: I have an alien character with a frill (like the Dilophosaurus is Jurassic Park), and I wanted a shape key for the frill to go down. I made the original mesh in ZBrush. I exported it from ZBrush and imported it into Blender (after making texture maps in Substance Painter). I exported the mesh from Blender and imported it into ZBrush. After I adjusted the mesh so his frill was down, I exported it from ZBrush back into Blender. When I tried to do the "Join as Shapes" option, the error occurred. 

Now, the workaround: I imported the un-adjusted, frill-up version of the mesh from Blender into ZBrush, then I exported it right back into Blender without making any real changes. Now both the frill up and frill down versions are both exported from ZBrush (from Blender). Not sure why, but now "Join as Shapes" works, so I'm happy. Since the vertex count never changed, the texture maps I made in Substance Painter still work too. 

Thanks!